How does 802.11ax (Wi-Fi 6) work?

By Michael Spalter
March 2021

About the author

Michael Spalter

Michael Spalter


Michael Spalter has been a networking technician for over 30 years and has been the CEO DrayTek in the UK since the company’s formation in 1997. He has written and lectured extensively on networking topics. If you’ve an idea for a blog or a topic you’d like explored, please get in touch with us.

Existing wireless LAN (Wi-Fi) standards, from 802.11b through to 802.11ac are to be joined by the latest standard, 802.11ax. In order to make these standards easier to recognise in marketing and use, new friendlier names have been adopted. 802.11ax is known as "Wi-Fi 6" and we'll use that term in this article.

Still the Same Channels

Wi-Fi frequencies are allocated by government within unlicenced bands.  The main bands used in Europe are the 2.4Ghz band (2401-2495 Mhz) and the 5Ghz band (from 5160-5730Mhz*) . When we used 802.11b, the 2.4Ghz band was divided into 13 channels of 20Mhz width each so 12 different base stations and clients could co-exist within the 94MHz frequency space, albeit with some overlap.

As the new standards developed, one reason that higher speeds were possible were simply by increasing the bandwidth used within the same allocated bands, so instead of a 20Mhz channel, you could use up to 8 times the size (160Mhz) but that used 8 'channels' which could be the whole channel range of some access points. As channels overlap and there will be other Wi-Fi devices using the same frequencies, it's often more effective or reliable to stick to narrower channels (80Mhz or 40Mhz).

802.11ax can use either the 5Ghz or 2.4Ghz band, and in 2020, Ofcom allowed more channels within the 5Ghz band to be used for regular Wi-Fi which were previously restricted by the DFS/TPC requirements.

*That is the standard channel range (36-64) but the band varies considerably by territory and whether DFS/TPC is permitted. Other channels may be permitted, but for specific uses only (indoor/outdoor, low power etc. You can learn more about DFS/TPC here. Some countries also have Wi-Fi bands in the 3.6Ghz and 900Mhz ranges. For further details see Wikipedia.

Wi-Fi 6e

In 2020 Ofcom in the UK as well as the American FCC announced that the 6Ghz band is to be opened up for unlicenced use, which will be usable for 802.11ax once chipsets are available to vendors.  Wi-Fi 6 products which can operate in the extended 6Ghz band will be known as Wi-Fi 6e.  Wi-Fi 6e is otherwise identical to regular Wi-Fi 6.

Capacity is the 'new metric'

Whereas previous Wi-Fi standards were measured by their theoretical peak speeds ('300Mbps', '1200Mb/s' etc.), due to the huge increase in device density, future Wi-Fi networks performance will be measured by capacity - the ability of a given network to reliably sustain a given number of clients and co-exist. For this reason, Wi-Fi 6 claims a potential 4x increase in capacity, not speed. This is why another name used for 802.11ax/Wi-Fi 6 is "High efficiency wireless".

Wi-Fi 6 does have a higher PHY rate vs. Wi-Fi 5 (802.11ac), which is 10Gb/s vs. 7Gb/s but more efficient use of that bandwidth will provide the most benefit. Whereas previous Wi-Fi standards were all about greater speed, Wi-Fi 6 is primarily about capacity. The 'spectral efficiency' of 802.11ax is measured as 62.5 bps/Hz.  That means that for every wave per second, we're getting 62.5 bits transmitted. 802.11ac is 40bps/Hz.  The use cases which will most benefit from Wi-Fi 6 will be locations with a higher number of client devices, such as shops, hospitality, shopping malls, offices and dense apartment buildings.

Comparing Wi-Fi 4,5 and 6

    802.11n 
Wi-Fi 4
  802.11ac (Wave2)  
Wi-Fi 5
802.11ax
Wi-Fi 6
Max Speed per Stream

172Mb/s
(40Mhz channel)

433Mb/s
(80Mhz Channel)
600Mb/s
(80Mhz Channel)
Encoding  OFDM OFDM OFDMA
QAM Encoding (max) QAM-64 QAM-256 QAM-1024
Spatial Streams (Max) 4 4 8
SU-MIMO  Y Y Y
MU-MIMO - DL Only UL & DL 
Subcarrier Spacing*  312.5Mhz  312.5Mhz 78.125Khz
Freq. Bands Used  2.4Ghz 5Ghz 2.4Ghz, 5Ghz
& 6Ghz
Subcarriers (@ 20Mhz) 64 64 256
Channel Width 20-40Mhz 20-160Mhz 20-160Mhz
FFT Points (Max)**  128 512 2048
OFDM Symbol Duration*** 3.6μs  3.2μs 12.8μs
Guard Interval /  Cyclic Prefix 0.4-0.8μs 0.4-0.8μs 0.8-3.2μs
BSS Colouring - - Y
Spectral Efficiency 15bps/Hz 42.5bps/Hz 62.5bps/Hz

*312.5Mhz is a 20Mhz channel split into 64 subcarriers by the FTT (Fast Fourier Transform) - see the later section on OFDM. **A Fourier Transform is the mathematical process of breaking down a complex signal into its component parts, in this case, the individual subcarriers. Narrower subcarriers require a much more accurate FTT engine, as provided by the Wi-Fi 6 chipsets. Longer guard intervals provide more robustness, particularly for Wi-Fi's higher QAM density. *** Ignoring guard intervals (Wi-Fi 4) and cyclic prefixes (Wi-Fi 5/6).

Wi-Fi 6 Maximum Speed

The claimed maximum PHY data rate of Wi-Fi 6 is "9.6Gb/s" (and Wi-Fi 5 is "3.5Gb/s"). If you've used a Wi-Fi 5 (802.11ac) device, you've never seen Gigabit throughput so how do we get those numbers and what do they really mean?

You can calculate the maximum data rate of a given Wi-Fi protocol by multiplying its characteristics and dividing by the duration of each symbol. For Wi-Fi 6, we can use the following numbers:

10      : Number of bits per point in 1024-QAM
2048  : No. of subcarriers of 78.125Khz in 160Mhz (1960 for data).
83%   : Coding Rate (this is how much of the PHY data stream can be used for actual data) (83%=5/6)
8        : No. of Spatial Streams
13.6   : Min. Symbol Duration (μs inc. guard interval)

The above numbers are maximum values, apart from the symbol duration (which is the minimum).

The calculation is:

(10 * 2048 * 0.83 * 8) / 13.6 = 9,569 = 9.607Gb/s total maximum data rate, including overheads/signalling.

Next, that shiny new Wi-Fi 6 wireless router you just bought which claims "11Gb/s". That's a lovely big number but it's not useful to gauge real-world performance. It could, arguably, be useful for comparing like-for-like (i.e. against another device's specification) but only if everything is measured identically and the marketing people didn't get carried away. So where does the "11Gb/s" come from?

It can be firstly broken down into 1.2Gb/s + 4.8 Gb/s + 4.8Gb/s which are the raw data rates on 'three' bands of 2.4Ghz and two identical bands of 5.8Ghz.  When vendors speak of a "triple band" Wi-Fi device, two of the bands are normally the same as each other; you just select a different channel.

The wireless router uses a 160 Mhz wide channel with 1024-QAM and 4 spatial streams.
That gives your 4.8Gb/s. 

In reality, a given environment will provide various impediments to making full use of these maximums, as well as normal interference, competing client devices, other nearby services and so on. Furthermore, you want to transmit as well as receive and whilst the access point or base might have many streams or antenna, more compact devices (your laptop or phone) will generally have far fewer which means the possible throughput to an individual device is limited. The actual wireless protocols and error correction also take up a significant part of the bandwidth.

However, whilst this explains where these 'huge' numbers come from in vendors' marketing materials or formal ITU-T specifications, it doesn't mean that Wi-Fi 6 isn't still effective and useful. In the real world with a good signal, a realistic expectation (as tested by various independent writers and magazines) indicates that a Wi-Fi 6 client in the 5Ghz band can achieve real-world throughput rates in excess of 1Gb/s in ideal conditions.  It's not 11Gb/s, but it's still fast and remember that Wi-Fi 6 isn't all about speed - speed does improve upon Wi-Fi 5, but the main benefits are in capacity, reliability and efficiency (see earlier).

 How does Wi-Fi 6 deliver its benefits?

We've explained that Wi-Fi6 provides benefits in speed - a claimed fourfold efficiency, whilst still using the same limited channels. All of this benefit is achieved by smarter use of the channels. Various new technical methods have been adopted to provide this efficiency:

  • 2.4Ghz and 5Ghz Band Usage

    Whilst 802.11ac (Wi-Fi 5) provided speed benefits over previous standards, it operated in the 5Ghz band only. The allocated 5Ghz band has fewer channels (depending on locale and DFS/TPC support) and as a higher frequency has lower range (signal penetration) than 2.4Ghz frequencies (higher frequencies are more absorbed by obstructions). Wi-Fi 6 can be used in either band so can benefit from the use of 2.4Ghz if longer range is required or there is less congestion in that band.

  • MU-MIMO

    MIMO (Multiple Input Multiple Output) MIMO multiplies the capacity of a radio link by using multiple antenna at both ends of the link. The properties of Multipath Propagation, a phenomenon of physics which is normally a problem for radio transmission can actually be exploited by using multiple transmitting and receiving antenna on the base and client device to transmit multiple signals on the same channel.

    When a transmitter has multiple antennae, a feature called beamforming can be used. Beamforming was introduced properly in Wi-Fi 5. Beamforming works by transmitting from multiple antenna, with the signal slightly delayed between each. The overlapping waves cause interference with each other. By using different antenna combinations, the signal at a given location will either be stronger (constructive interference) or weaker (destructive interference). The best antenna combination can then be used for each different client. Beamforming is an important part of MU-MIMO.

    MU-MIMO (Multi User Multiple Input Multiple Output) extends MIMO further by allowing multiple users (devices) at the same time.

    Whilst a Wi-Fi 6 base (access point) might support up to 8 spatial streams, the clients (phones, laptops, TVs) may only use one or more antenna, supporting only a single stream each. The number of antenna isn't necessarily indicative of the number of streams and multiple antenna may also be used to provide signal diversity. Wi-Fi 5 supports MU-MIMO though it wasn't widely adopted, is limited to 4 streams and is available in the DL stream only. With regular MIMO, even where a wireless base (Access Point) supported 4 streams if a client (particularly a phone which has limited internal space) only had 1 or 2 Wi-Fi antenna, it couldn't take advantage of MIMO.

    In Wi-Fi 6, MU-MIMO is available in both UL* and DL* streams and there can be to 8 streams, supporting multiple client devices at once.

    *DL is download (respective to transferring data from wireless base to client) and UL is Upload (from client to base).


  • Higher Density Sub-Carrier Spacing

    In Wi-Fi 5, the available bandwidth is split into 256 subcarriers (individual frequencies), 312Khz apart. In Wi-Fi 6, there are 1024 subcarriers and they are spaced 78.125Khz apart, so 4 times more dense. In Wi-Fi 6, the OFDM symbol duration (the baud rate) is actually longer; this is because symbol rate and subcarrier density have an effect on each other - a higher symbol rate causes signal to spread into adjacent frequencies and in increase in EVM (Error Vector Magnitude). Interestingly, in G.Fast (the DSL line system) they did the opposite - increased the symbol rate and dropped the subcarrier density.

  • OFDMA

    This is perhaps the most important feature in relation to efficiency and best use of bandwidth.

    OFDM
    ODFM

    OFDMA
    OFDMA



    Previous Wi-Fi standards used a method called OFDM (Orthogonal Frequency-Division Multiplexing) to encode data into multiple carrier frequencies (subcarriers). As an example, 'Channel 1' in the 2.4Ghz band runs from 2401–2423Mhz (assuming 20Khz channels). 802.11ac (Wi-Fi 5) uses 52 subcarriers for data (plus 12 more for signalling so total 64) so each subcarrier has a different frequency in the 2401–2423Mhz range, each spaced 312.5Khz apart ( 312.5Khz * 64 subcarriers = 20Mhz!).

    In OFDM, each subcarrier is orthogonal to the next (its phase is at 90 °). That orthogonal arrangement reduces interference from adjacent subcarriers so that a guard band is not required between carriers which would otherwise take up space.  OFDM is effectively the same as DMT; DMT being the commonly used name when this system is used in wireline systems (DSL etc.)

    Wi-Fi 6 uses OFDMA (Orthogonal Frequency-Division Multiple Access) which is a more efficient method than plain OFDM. OFDMA has been used to good effect on 4G cellular networks.  It takes OFDM and subdivides each subcarrier into smaller units, called Resource Units (RUs). So whilst, in OFDM, each of the 1024 subcarriers can carry data for only one client at a time and any spare space is wasted, with OFDMA, each subcarrier can carry data for multiple clients.  RUs are allocated to clients and coordinated by the Wireless base (AP/router) however an AP may also define a general pool of random access RUs which any client can grab ('randomly') if wants to transmit and doesn't have an RU allocated. It's down to 'luck' that another device doesn't use the same RU at the same time.

    Using OFDMA is analogous to a full shipping container carrying payloads for multiple clients compared to shipping multiple containers, each of which will only be partially full. A partially full payload is far less efficient due to the fixed overheads of preamble and packetisation, particularly if the payload is small. In OFDMA, those overheads are amortized between multiple users.  

    As one frame is able to carry data for multiple clients, they also share a single frame header for that frame. Headers add overheads, so less headers means less overhead.  In Wi-Fi 6, there are now actually 4 types of High Efficiency (HE) Physical (PHY) header to suit different frame types - Single User (HE SU), Multi-User (HE MU), Single-User Extended-Range (HE SU-ER) and Trigger Based (HE TB). Trigger Frames are an essential part of Wi-Fi 6, used for AP-to-client telemetry, but beyond the scope of this article.

    OFMDA has an additional advantage for IoT devices which require a very low power consumption - a subdivision of a subcarrier means that very low data rates can be used and only the necessary data is transmitted, which saves power.

  • Scheduled UL Access

    In previous Wi-Fi standards, opportunistic sending was applied. This means that a station/device would listen and if it thought the channel was clear, it would try to send its data but if another device did that at the same time, there would be a collision, the transmission failed and would try again. Where an access point (Wi-Fi base) applies UL scheduling, it's equivalent to putting a crossroads and traffic light instead of a busy roundabout. You might have to wait, but when it's your turn, you move at full speed. With Wi-Fi the wait would be milliseconds of course, not minutes. Scheduled UL access can be applied to MU-MIMO and OFDMA.

  • 1024-QAM

    Provides up to 25% higher data rate by having 10 bits encoded per symbol vs. Wi-Fi5 which gives a raw throughput of 600Mb/s (per channel) vs. 433Mb/s max in Wi-Fi 5 (assuming an 80Mhz 'channel'). The ability of higher order QAM to operate does depend on signal quality. See the later section on how QAM works. QAM is an elegant and fascinating method and I'll be explaining how it works in a later article.

  • Target Wake Time

    Target Wake Time (TWT) is useful for battery powered or mobile devices where power conservation is an issue or desirable. TWT enables a device to determine how often they will need to wake up and send or receive data. IoT devices, in particular, may have set cycles whereby they only need to send data infrequency (consider a smart meter which sends its data only once a day). The central access point can negotiate or define set access times and put a device to sleep for up to 5 years. This provides more efficient use of the bandwidth as multiple devices can avoid overlapping time windows.

  • BSS Colouring

    Previous Wi-Fi standards expected every user to choose their own Wi-Fi channel and most often this was set automatically by your Wi-Fi base (or Wi-Fi router). Whilst some bases might do a scan to check for congestion at the time of setup, that doesn't take account of real client density or traffic over time. Further, there was no cooperation or communication between bases. The result of this is that when a neighbouring wireless network is transmitting and your wireless network's clients 'hear' that, they wait until that network stops transmitting before they can begin communicating.

    With Wi-Fi 6, a system called BSS (Basic Service Set*) Colouring is adopted whereby each wireless network's traffic is marked with a 'colour' (actually a number). The colour marking enables devices to identify an overlapping wireless network (or OBSS) on the same channel so that they can ignore its communication if it's far enough away. If the BSS colour is the same, there's no overlapping and clients will wait for other clients on 'their' network to finish communicating before they start, to avoid collisions. There is also additional intelligence using a Network Allocation Vector (NAV) to predict future traffic. An access point (wireless base) can use this information to choose a less congested channel, and continue to vary this in real time as the environment changes.

    *A 'Basic Service Set' is set of Wi-Fi devices connected to an Access Point (Wireless Base).

  •  Outdoor Usage Improvements

    Wi-Fi 6 includes features to benefit outdoor applications. Whilst there are less obstructions in outdoor use cases, there are other complications, notably doppler reflections which can be caused by fast moving objects. This can cause channel variation errors on received packets. To reduce the effect of this, redundancy (known as midamble) is built into the protocol, notably the repeating of the HE-LTF and HE-SIG-A fields throughout packets.  This enables a receiver to keep track of a time-varying channel.

  • Guard Intervals

    A guard interval is period immediately before each symbols - a time gap or delay. This reduces Inter-Symbol Interference (ISI) where symbols are so tightly packed into QAM. ISI can be caused by reflections, echos and propagation delays.  Previous Wi-Fi standards already used guard intervals, but Wi-Fi 6 now has three alternative lengths (0.8us, 1.6us and 3.2us) to allow for the higher level of QAM (1024). Guard intervals are applied depending on the level of local interference or signal quality. A longer guard interval improves signal fidelity at the expense of raw speed, however, if a poorer signal with a shorter guard length causes packets to have to be resent, the speed would be slower than if a longer guard interval was set.  The guard interval is normally set automatically by the devices, not the user.

Legacy Protocols & Devices

To gain the maximum benefit of all of the new methods provided by Wi-Fi 6, you really don't want to have legacy protocols around which don't respect the new protocols. 802.11ac and previous technologies aren't aware of the smart telemetry and new methods of Wi-Fi6, so they'll ignore them - they will carry on inefficiently using the channels, ignoring the pleas of your new 802.11ax devices. So, for the maximum benefit of Wi-Fi 6, you need to replace all non-Wi-Fi 6 devices - your own and also any neighbours and remove any older devices, so not economically practical in the short term. That doesn't mean, however, that Wi-Fi 6 mixed environments don't provide a benefit. Some of the new technologies adopted will certainly provide material benefits in mixed environments. Wi-Fi 6 is backward compatible - it maintains the legacy preamble in it's 802.11 frames, and Wi-Fi 5 devices will benefit from additional spactal streams.

I hope you enjoyed this article and learning about 802.11ax.  As always, please let us have your comments below and any suggestions for new articles.


Tags

802.11ac
802.11ax
WiFi 6
Wi-Fi 6
Wi-Fi 6e
ISI
Guard Interval
Wifi 5
OFDMA
QAM
1024-QAM
OFDM
Wi-Fi 6
BSS
Wi-Fi 5